There is an interesting article on Beliefnet’s blog, Beyond Blue: A spiritual journey to mental health, in which Terese J. Borchard attempts to draw a line between the experience of loss and spiritual dryness which John of the Cross described as the Dark Night of the Soul and clinical depression.
This is difficult in many ways, especially given that, as Borchard notes, one can experience both at the same time. Her argument amounts to the fact that a religious person in the dark night is well aware of the trial that they are undergoing and may even be invigorated by the spiritual trial, while someone who is depressed simply seems depressed.
However that may be, even a casual reading of The Dark Night of the Soul, reveals an almost unending list of sufferings–physical, emotional, and spiritual–that seem a lot like depression, and, in terms of the symptoms, are indistinguishable.
My argument would be that Saint John’s path is a way of using deep depression to find God and to refine one’s spirituality. Any depression has the potential to become a Dark Night of the Soul if and when a person ceases to run from depression and accepts it as revealing a truth about the emptiness of self and the difference between the self and God.
From a psychoanalytic point of view, the best work on this is Julia Kristeva’s Black Sun, a work which links and compares mysticism and depression.
Like Borchard and Kristeva, I do believe that medication can be invaluable for the treatment of depression. I simply also would argue that depression can teach us deep truths about the universe.